Like I noted in my response to your previous post when I referred to p 52 in the document you referred to, you are wrong about the actual rear strength in the newer models.
Hmm.....my replies have been pointed more to available payloads for the OP which is controlled by rear axle suspensions which both trucks have the same RAWR numbers...same Dana 300 M series that for several years have been 9000 up to current 9900 rwar.
Now for actual strength more splines and larger axle shaft diameters can help but axle splines and shaft diameters don't carry weight on a full floater rear axle.
Now if were talking about the older F450 pickups when Ford gave them a 11500 RAWR then of course those model years the F450 had much more payload capabilities.